Conventional vacuum-operated brake power boosters as a rule have a housing in which there is an axially movable wall which is adapted to be reciprocated within the housing, the wall dividing the inner space of the housing into two chambers. By means of the two chambers, a pressure difference can be generated on the opposite sides of the movable wall. The pressure difference results in the movable wall being shifted from the higher pressure in the direction of the lower pressure. A vacuum-operated brake power booster of this type furthermore comprises a control valve which is actuated through the brake pedal and which controls the amount of vacuum acting on the one side of the axially movable wall. As a rule, the axially movable wall is connected to the brake master cylinder of a hydraulic braking system of the automotive vehicle in such a manner that when the pressure difference has its effect on the movable wall causing movement of the movable wall, brake fluid is displaced from the brake master cylinder into the hydraulic braking system to apply the wheel brakes of the vehicle.
In the design of braking systems for automotive vehicles, it is expedient to configure the vacuum-operated brake power booster in such a manner that the brake response phase during which the vehicle driver moves the brake pedal and interrupts the connected of flow between the two sides of the axially movable wall by means of the control valve is reduced to a minimum.
A brake power booster of the kind generally shown herein is known from German printed and published patent application 3,042,096 Al. In that reference, a wedge is provided which is seated with play both in the control valve piston and in the control valve body. The wedge is adapted to come into abutment with the housing of the brake power booster in such a way that in the release position of the brake, the vacuum valve seat is lifted a minute distance from the poppet valve. That known brake power booster has the disadvantage that the potential lost motion cannot be completely reduced to zero.